I really need help with this, please help

As the property of tangent line of circle, the 2 right triangles ABC and ADC are congruent.

=> AB = AD or j = k

j = 25x^2 + 100x + 9

k = 21x^2 + 100x + 18

=> 25x^2 + 100x + 9 = 21x^2 + 100x + 18

=> 4x^2 = 9

=> x^2 = 9/4 = (3/2)^2

=> x = 3/2 ( -3/2 value is not valid)

Hope this helps!

:)